In the handling of flowing high pressure fluids, it has been customary to utilise orifice means having a high velocity short throat section to attain energy losses or high pressure drops. If the fluid is in a liquid state and liable to flash, that is, vaporise or turn to a gaseous condition on the downstream side of the orifice or valve opening, it may condense implosively and induce damaging shock waves, cause erosion, and the like. Also, as the velocity of the fluid in the valve exceeds the velocity of the fluid in the line, several disturbing reactions occur. A most serious problem is rapid erosion of the valve surfaces by direct impingement of the liquid and any foreign particle suspended therein. Additional erosion results from cavitation. Cavitation may be defined as the high speed implosion of vapour against those internal parts of the valve controlling flow (the valve trim) and the valve body.
In addition to the severe problems resulting from erosion, the increased velocity also causes the flow characteristics of the valve to become unpredictable and erratic.
Other problems created by the high fluid velocity in the valve are severe noise generation, trim fatigue and possible degradation of flowing fluid materials such, for example, as polymers.
Fluid-borne noise downstream of control valves is often very high. If not treated or contained with the pipe, this noise can result in sound pressure levels of 110 to 170 dB three feet from the valve exit. Sound sources of this magnitude are hazardous to personnel and frequently result in complaints from local residents.
Mufflers and silencers can typically only attenuate fluid-borne noise 20 to 30 dB. Therefore, only partial success has been achieved with them in obtaining desired sound pressure levels.
Furthermore, a typical path treatment system ie, the muffler, lagging support structure etc is very cumbersome and expensive, often, the total cost of path treatment for noise can exceed the valve cost many times over.
In order to overcome or ameliorate the above problems, there have been introduced devices which effect energy losses in high pressure fluids without increasing velocity and shock wave reaction by sub-dividing the flow into a plurality of small, long passageways with abrupt turns creating friction and pressure drop in the fluid, thus avoiding damage and erosion in the equipment. Such a device is disclosed, for example, in U.S. Pat. No. Re. 32,197. There, the passageways are provided in an annular stack of separate members having abutting faces enclosing a plurality of individual passageway grooves which are angular between the inlet and outlet of the stack to turn the fluid and to provide a substantially longer flow length than between the inlet and outlet ends of the stack. The stack is mounted in the fluid passage of a valve housing and a valve plug movable within the annular structure controls the number of passageways in the stack through which the fluid can flow.
A modified device of this type is disclosed in GB-A-2,273,579 in which at least one passageway in the stack of members of discs includes a void between the inlet and outlet region of the disc, the void expanding the cross-sectional area of the energy loss passageway.
Valves incorporating a flow control device including a stack of discs having energy-loss passageways have become very successful commercially and it is an object of the present invention to provide an improvement in devices of this type.